The present invention relates to a solenoid valve provided in a hydraulic circuit of an automotive vehicles or the like to control the hydraulic pressure thereof.
As disclosed in a Japanese Unexamined Utility Model Publication No. Sho. 64-15882, it is well-known to use a solenoid valve in which a spool valve is movably inserted into a sleeve so as to be operated by the excitation of an electromagnetic coil for the purpose of switching hydraulic pipe lines.
FIG. 8 shows an example of such conventional solenoid valve. The solenoid valve 1 comprises a spool valve 3 movably inserted into a sleeve within a casing 2. Tapered first and second valve seat portions 3a and 3b are formed on an outer surface of the valve 3. On the other hand, first and second valve seat 4 and 5 are formed on an inner circumferential surface of the sleeve so that the first and second valve seat 4 and 5 can be brought into contact with the first and second valve seat portions 3a and 3b, respectively.
A core 6 is fixed to a predetermined portion above the valve 3 within the casing 2, and is provided on the upper surface thereof with a attraction surface 6a. A plunger 7 is provided in a central portion of the casing 2, formed with a surface confronted with the attraction surface 6a, and moved in a central portion of the core 6 so as to depress the valve 3. A bobbin 8 made of insulative material is fixed to the circumference of the core 7. A coil is mounted and wound around the bobbin 8.
A lid member 10 is fixed to an inner surface of the casing 2 near the upper end portion of the casing 2. The valve 3 is biased at its lower end by a spring 11 in a direction toward the plunger 7. The spring 11 is supported at its lower end to a bolt 12 threadingly engaged with the casing 2 so that the biasing force of the spring 11 can be adjusted by the rotation of the bolt 12.
In the solenoid valve thus constructed, if the coil 9 is not in an excitation state, a gap XG is formed between the attraction surface 6a of the core 6 and the surface of the plunger 7 by the biasing force of the spring 11. In this state, the first valve seat portion 3a of the valve 3 is away from the first valve seat portion 4, whereas the second valve seat portion 3b contacts the second valve sheet 5.
If the coil 9 is energized from this case, that is, if the coil 9 becomes into the excitation state, the surface of the plunger 7 is sucked to the attraction surface 6a of the core 6 due to the magnetic force. As a result, the valve 3 is depressed against the biasing force of the spring 11 so that the first valve seat portion 3a contacts the first valve seat 4 whereas the second valve seat portion 3b is away from the second valve seat 5, thereby effecting the switching operation of the hydraulic pipe lines.
For the above-mentioned solenoid valve 1, it is an important factor that the depressing force of the plunger 7, which is acting on the valve 3, is appropriately controlled or set. In order to control or set the depressing force, it is necessary to accurately control or set the gap XG between the plunger 7 and the attraction surface 6a of the core 6. However, due to an inaccuracy of the each member of the above-mentioned solenoid valve, the gap XG is likely to be smaller or larger than a desired distance, so that it is difficult to provide a solenoid valve with the gap XG formed at a constant distance. As a result, the entire performance of the solenoid valve is deteriorated. That is, it is difficult to provide the solenoid valve with a constant gap distance and desired performance cannot be obtained.